Herded cattle and wild grazers partition water but share forage resources during dry years in East African savannas
Introduction
African savannas hold the earth’s greatest diversity of wild large-herbivore species and have sustained these multi-species assemblages for millennia (du Toit and Cumming, 1999). However, a steady reduction in wild herbivore numbers and diversity has been observed for the past 20 years, attributed to a combination of factors, such as an increase in drought periods, land use change, poaching (Ottichilo et al., 2000a, Ottichilo et al., 2000b), and competition for forage resources between wildlife and livestock species (Prins, 1992, du Toit and Cumming, 1999, de Leeuw et al., 2001, Said, 2003). The potential competitive interactions between native wild herbivore species and introduced livestock species have large conservation implications since the combination of transhumance pastoralism and wildlife conservation is common across most African savannas (Prins, 1992, Prins, 2000, du Toit and Cumming, 1999). Global concerns on the competitive effects of livestock on wildlife have increased in a variety of ecosystems (Prins, 1992, du Toit and Cumming, 1999, de Leeuw et al., 2001, Mishra et al., 2002, Said, 2003, Madhusudan, 2004, Steinfeld et al., 2007, Fankhauser et al., 2008, Yoshihara et al., 2008) and have led to diverse management actions, from creating more wildlife protected areas where livestock grazing is reduced (Mishra et al., 2002, Madhusudan, 2004) to community based management where local people are involved in, and benefit from, wildlife conservation (du Toit and Cumming, 1999, Yoshihara et al., 2008), or a combination of both (Georgiadis et al., 2007a).
The extraordinary diversity of wild herbivore assemblages in African savannas has been attributed to resource partitioning among herbivores in combination with high primary productivity and evolutionary history (Jarman and Sinclair, 1979, Murray and Illius, 1996, Prins and Olff, 1998, Olff et al., 2002). Body weight is a key species trait involved in resource partitioning and assumed to be the crucial factor determining inter-specific interactions (Prins and Olff, 1998). Many aspects of an animal’s morphology, physiology and ecology vary allometrically with body weight (Western, 1979, Owen-Smith, 1988), from gut capacity and bite size, to food intake rate and feeding site selection (Owen-Smith and Cumming, 1993, Prins and Olff, 1998, Voeten and Prins, 1999). Body weight ratio is therefore used as an indicator of potential resource competition, expected to be strongest among herbivore species of similar body weight (Prins and Olff, 1998).
Introduced livestock species can overlap in resource use with wild herbivore species, because they have similar resource requirements to these wild herbivores, and they only share a relatively recent history of co-occurrence. This is thought to prevent resource partitioning with them (Fritz et al., 1996, Voeten and Prins, 1999, Prins, 2000). Competition for forage resources between two species is unavoidable if (1) their habitats overlap, (2) their diets overlap, and (3) their resources are limited (Wiens, 1989). There are some indications of both habitat (Lamprey, 1963, de Boer and Prins, 1990, de Bie, 1991) and diet overlap (Casebeer and Koss, 1970, Field, 1975, Hoppe et al., 1977, van Dyne et al., 1980, Voeten and Prins, 1999) between wild grazers and cattle. Wild grazers similar in body weight to cattle have a higher potential of resource competition with cattle, implying that these grazers are the ones most likely to be reduced in numbers and displaced by cattle (Prins and Olff, 1998, Mishra et al., 2002). Indeed, Voeten and Prins (1999) found that the resource use overlap between cattle and the comparably large zebra and wildebeest could result in resource competition under food-limited conditions. Although evidence for resource limitation has remained difficult to demonstrate in the field (Prins, 2000), extended dry seasons are known to negatively influence wild herbivore populations through reduced reproductive and survival rates, as during these dry periods low grass productivity probably limits the energy uptake of grazers (Sinclair, 1977, Prins, 1996, Ottichilo et al., 2000a). Hence, resource limitation is probably most acute during low rainfall periods (in accordance with Young et al., 2005, Georgiadis et al., 2007b).
Savannas are social–ecological systems where pastoralists play an active role, especially in non-protected wildlife areas (Walker et al., 1981, Walker, 1993, Ludwig et al., 1997, Anderies et al., 2002, Georgiadis et al., 2007a). It is likely that pastoralist decisions on cattle distribution can influence the potential for resource competition with wildlife in these areas. Water availability during dry periods is an important factor structuring the livestock grazing system in arid (de Leeuw et al., 2001) and semi-arid savannas (Western, 1975, Bergström and Skarpe, 1999, Coppolillo, 2000). During periods of limited water availability, pastoralists tend to move their water-dependent cattle towards remaining water bodies (Western, 1975, Coppolillo, 2000, de Leeuw et al., 2001). Cattle concentration may cause local overgrazing, i.e., changes in plant structure and composition and habitat degradation around these water bodies (Andrew, 1988, Johnson, 1993, Pickup et al., 1998).
In this paper, we analyse the distribution of wild grazers in relation to cattle abundance and distance from water bodies during distinct periods of rainfall, between 1983 and 2000, in a Kenyan savanna, representative of semi-arid savanna ecosystems in East Africa (Pratt and Gwynne, 1977, Sombroek et al., 1982). We analyse the distribution in relation to both permanent (i.e., lakes and rivers) and seasonal water bodies with a more erratic availability, depending on local rainfall and topographic conditions (i.e., temporary streams, swamps and lakes), as well as proper maintenance (i.e.,, man-made pools and boreholes) (Dietz et al., 1986, de Leeuw et al., 1991). In this savanna ecosystem the transhumance Maasai pastoralists live mainly in temporary settlements, and move their cattle according to environmental conditions (Western, 1973). We hypothesize that the potential of competition for forage resources between cattle and wild grazers, and the displacement of wild grazers by cattle, is highest close to permanent water bodies during dry periods due to cattle concentration around these water bodies. More specifically, we predict that (1) during a resource limited period (i.e., low rainfall period) cattle are concentrated in areas close to permanent water bodies, and wild grazers in areas far from these water bodies as a result of, or to avoid, resource competition, (2) during a non-resource limited period cattle and wild grazers are more homogenously distributed across the landscape, (3) wild grazers that do occur close to permanent water bodies during a resource limited period tend to be more dissimilar in body weight to the cattle present there. However, we also expect seasonal water bodies – available during part of the dry period only – to play a role in the distribution of cattle and wild grazers. We hypothesize spatial partitioning in the use of seasonal water bodies between the two groups of herbivores, so wild grazers are able to fulfil their drinking water requirements and reduce the potential of competition for water resources. In order to test this expectation, we quantified the association between grazer species and type of seasonal water body.
Section snippets
Study area
Kajiado district is a semi-arid district in Kenya located in the Rift Valley Province (between 1°0′–3°0′S and 36°5′–37°5′E) covering an area of 21,852 km2 (Fig. 1). The general topography is characterized by vast plains with occasional volcanic hills and valleys (de Leeuw et al., 1991, Campbell et al., 2005). Due to low and erratic rainfall almost 93% of the district is rangeland and only 7% has some potential for rain fed cropping (de Leeuw et al., 1991). The average annual rainfall varies
Spatial distribution of cattle and wild grazers
Our results show a significant departure from our first prediction. Cattle occurred in higher densities far from water during the resource limited period (Table 1), suggesting that during dry years the potential of displacement by cattle was higher far from water for most of the wild grazers. Indeed, we found the (wild grazer/cattle) ratios to be lower at intermediate and far distances from permanent water bodies for all wild grazer species, except kongoni and oryx (Table 2 and Fig. 3). In
Discussion
Wild herbivore decline in African savannas has been attributed at least partially to competition for forage resources with livestock (Ottichilo et al., 2000a, de Leeuw et al., 2001, Said, 2003). Competition between wild grazers and cattle is generally assumed (Lamprey, 1963, Casebeer and Koss, 1970, Field, 1975, van Dyne et al., 1980, de Bie, 1991), especially during low rainfall periods (de Boer and Prins, 1990, Voeten and Prins, 1999). During these periods cattle are often concentrated around
Acknowledgements
We thank Mr. Jaspat L. Agatsiva, Director of the Department of Resource Surveys and Remote Sensing, Kenya, and Jan de Leeuw for providing us with data. Evert-Jan Bakker, Thomas Groen, Anna Treydte, Pim van Hooft, Honoré Biaou, Johan du Toit and three anomymous reviewers are acknowledged for their helpful comments. We thank Herman van Oeveren and Pat van der Jagt for their help on the figures. A grant from Fonds voor Onderzoek ten behoeve van het Natuurbehoud (FONA) supported the research of the
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Present address: ETH Zürich, Institute for Integrative Biology, Universitätstrasse 16, CHN G 19, 8092 Zürich, Switzerland.